The present invention relates to water-in-oil emulsions and associated methods. More specifically, at least in some embodiments, the present invention relates to water-in-oil emulsions that comprise an oil-based continuous phase and a discontinuous phase in which at least a portion of the discontinuous phase droplets are hydrogel droplets, and associated methods.
Emulsions usually comprise two immiscible phases. The two immiscible phases usually include, but are not limited to, a continuous (or external) phase and a discontinuous (or internal) phase. The discontinuous phase usually exists in droplets in the continuous phase. The terms “droplet” and “droplets” as used herein refer to discrete portions of the discontinuous phase in the continuous phase, and do not imply any particular shape, size, or other characteristic of the droplets themselves. Two varieties of emulsions include, but are not limited to, oil-in-water and water-in-oil. Oil-in-water emulsions usually include, but are not limited to, a fluid at least partially immiscible in an oil-based fluid (usually an aqueous-based fluid) as the continuous phase and an oil-based fluid as the discontinuous phase. Water-in-oil emulsions are the opposite, having the oil-based fluid as the continuous phase and a fluid at least partially immiscible in the oil-based fluid (usually an aqueous-based fluid) as the discontinuous phase. Water-in-oil emulsions may be also referred to as invert emulsions.
Such emulsions have been used widely in oil and gas applications. For instance, emulsions may be used in the oil and gas industry for subterranean treatment applications, including drilling, production, and completion operations. Water-in-oil emulsions may be used because they may have superior performance characteristics when compared with water-based drilling fluids in some situations, e.g., when there is an abundance of water reactive materials in a well bore. These superior performance characteristics may include, e.g., better lubrication of the drilling strings and downhole tools, thinner filter cake formation, and better hole stability.
One problem that may be encountered when using water-in-oil emulsions is that such emulsions typically may undergo natural degradation processes, including droplet coalescence and Ostwald ripening, until the two phases, which may be at least partially immiscible at least initially, separate and the emulsion no longer exists. Having an unstable water-in-oil emulsion may be problematic because if the emulsion destabilizes, it may not have consistent, reliable properties. This problem may be exacerbated by the physical forces that the water-in-oil emulsion may undergo when being used in subterranean applications, such as thermal, mechanical, and chemical stresses. Surfactant-based emulsifying agents may be useful in water-in-oil emulsions for emulsion stabilization, especially when used in subterranean applications. However, such surfactant-based emulsifying agents may not always be effective at combating the tendency to coalesce, especially at elevated temperatures as often encountered in subterranean applications.
Water-in-oil emulsions often include, but are not limited to, a surfactant-based emulsifying agent. It is believed that traditional surfactant-based emulsifying agents may have a propensity for droplet coalescence, which may be undesirable. Surfactant-based emulsifying agents typically are thought to form a meta-stable structure around the internal water droplets, in which the hydrophilic headgroups are in contact with the water phase while the hydrocarbon (hydrophobic) tails are in the continuous oil phase. This meta-stable structure at the water/oil interface may have defects as surfactant molecules continuously diffuse in and out. Such defects may increase the water/oil interfacial tension and promote water droplet coalescence, which may result in eventual emulsion instability. This coalescence may be especially problematic in subterranean formations when these emulsions are subjected to formation fluids that may be present in the formation.
The aqueous droplets of the discontinuous phase may be subject to undesired coalescence and flocculation, which may lead to droplet size growth. This growth may be especially problematic in subterranean drilling operations because this growth may lead to flipping of the emulsion, resulting in a change to an oil-in-water emulsion. Flipping of water-in-oil emulsions may be detrimental to a subterranean operation not only because of the drastic changes in fluid properties, but also because it may introduce a water phase in direct contact with the formation. This may be undesirable as it can lead to swelling of the clay and possibly formation damage. Direct contact with the water phase that often has high salinity also may be likely to increase the rate and magnitude of corrosion upon downhole equipment. Additionally, unintended water droplet coalescence could also result in localized changes in oil/water ratio, viscosity properties, fluid density, solids settling, loss of stability of the emulsion, and create a greater temperature sensitivity of the emulsion.